I'm going back to school in January and will probably drop my KU membership. I still have a long WL there but I have 2 paid out of state library cards too and won't have as much reading time so the KU has to go for now at least. 

Anyway, a few times now I have been able to pay a small fee to upgrade a borrowed library book to an audible book and it was mine to keep after the library book expired.  I was wondering if the Kindle Unlimited audiobook upgrades work the same way. A couple of the books I checked out have cheap upgrade fees and I wouldn't mind paying 1.99 to own the audiobook.  I mean that's really cheap.  I'm tempted to try it with one, pay the upgrade and then return the book to see if it still works. But figured I'd ask here if anyone knew first.

Yes it should work the same way. I haven't tried it, although I've done it with library books multiple times.

Although.....I just looked at a book I have checked out in KU, and I get this notice:

Whispersync for Voice

Since you own both the Kindle book and Audible narration for this title, you can switch back and forthbetween reading and listening. Or, you can find audio companions for other Kindle books you own.

As another test, I just checked out a book, and I get the same message, even though I haven't prompted it to open the audio book or download it. So I don't think it's going to work.

Well obviously if I drop Kindle Unlimied, I won't have access to the ebook to switch back and forth. All I care is that I have access to the audiobook that I paid extra to upgrade. I might try it on one just to see before I cancel my subscription.  (meaning, buy the audible upgrade and then return the kindle unlimited book.)

I asked on the Kindle forum and consensus is that if I paid for the upgrade and return the ebook, the audio is still mine.

Well what I was saying is that it wouldn't let me buy the book. But that is probably for the books that have "narration added" instead of adding the paid audio upgrade.

It's an old post, but I thought I would update.  It's a crap shoot.  Some books allow you to upgrade for a small fee, but many of those are going as authors & publishers of audiobooks renew their contracts with Amazon.  As one author reported, he paid $12,000 out of pocket to have an audiobook made of his book and he gets $1 instead of $7-8 as he was expecting, per sale.  Even selling hundreds more does not make up for the loss.  His next audiobook, priced the same as his dead tree book at $12.99 had only been out for 3 months and it already cleared expenses, something he is still waiting on for now almost 2 years for the first one that was so heavily discounted. So, go for it while it's still available, but I think this marketing tool is on its way out in its current form.
 

$12,000 is the fee to have an audio book produced? Wow. Do you know what book it was? I bet that is on the high end of the fee scale.

One google search came up with this 

An average novel is about 10 hours of narration (roughly 90,000 words) and is likely to cost $200 or more per finished hour. 

So a 10 hour book would be $2,000 going by that rate. Of course that could vary upward, but that's a far cry from $12,000. Maybe the author quoted above was quoted wrong.